Rotolining vessels to coat the interior surface of a vessel with a desired thickness of fluoropolymer is known, as disclosed in J. Scheirs, Modern Fluoropolymers, John Wiley & Sons (1997). In such a process, a sufficient amount of fluoropolymer in powder form is added to a metal vessel to coat the interior surface of the vessel with the desired thickness of the fluoropolymer, followed by rotating the vessel about one or more axes in an oven, to melt the fluoropolymer, whereby the fluoropolymer covers the interior surface of the vessel and forms a seamless lining. This fluoropolymer lining protects the vessel from corrosive materials stored or handled by the vessel, by virtue of the chemical inertness of the fluoropolymer forming the lining and of the lining being continuous with respect to the interior surface of the vessel that would be exposed to the corrosive materials if the lining were not present. Thus, the lining is free of holes, even pinholes, through which the corrosive material could penetrate the lining to attack the material of construction of the vessel. Adherence to fluoropolymer surfaces are rare because of the non-stick property of fluoropolymer, so such linings are not subject of deposition of components of conveyed materials, as may occur if the conveyed material cools in transport, causing less soluble components to come out of solution. Similarly, materials conveyed in the lined vessel are not exposed to metal making up the vessel and are not thereby contaminated. The materials contact only the inert fluoropolymer surface of the lining. However, rotolining can be costly because the oven which is used for this process is expensive, particularly when the vessel must be rotated about multiple axes.
Certain melt-processible fluoropolymers, such as copolymers of ethylene with either tetrafluoroethylene (ETFE) or chlorotrifluoroethylene (ECTFE), on rotolining, form linings which adhere to the interior surface of the vessel without the used of primer or adhesive. However, for the perfluorinated melt processible copolymers, such as tetrafluoroethylene/hexafluoropropylene (FEP) and tetrafluoroethylene/perfluoro(alkyl vinyl ether) (PFA), the lining does not adhere so well to the vessel, instead forming a “loose lining”. Loose linings are held in place by the configuration of the interior surface of the vessel, i.e., mechanically locked into place. While this is satisfactory in some applications, the lack of adhesion between lining and interior surface of the vessel becomes disadvantageous in such vessels as pipes, where the opportunity for mechanical restraint on movement of the lining is limited, especially as the length of the pipe increases. Moreover, the passage of material, such as oil through the pipe, especially when the flow of the material varies, subjects the lining to vibration and mechanical stress, which can cause the lining to crack and fail. For corrosive materials, lining failure exposes the vessel to corrosion. For materials in which maintenance of purity is critical, such as food processing, pharmaceuticals and semi-conductor processing, where contamination of the medium being conveyed by exposure to the material of construction of the vessel (i.e., metal), caused by such cracking and failure, is unacceptable.
Furthermore, with loose linings, gas can diffuse into the space between the lining and the vessel wall, pushing the lining away from the all and restricting material flow. This can lead to blockage and interrupted flow, which are undesirable.
Methods of lining pipes with preformed films are known, see for example, U.S. Pat. No. 2,833,686 to Sandt and Research Disclosure No. 263060, which describe liners made of polytetrafluoroethylene, which is a non-melt-processible fluoropolymer. Both of these references use a fluorinated ethylene propylene bonding agent, which does not provide particularly good adhesion because of the non-stick properties of fluoropolymers generally.
A fluoropolymer preformed liner for a pipe is disclosed in U.S. Pat. No. 3,462,825 to Pope. No bonding agent is used in this patent. Consequently, pressure and temperature cycling that may occur in the use of such lined pipes can cause the lining to buckle, pulling away from the interior surface allowing accumulation of gases and liquids between the liner and the wall surface and narrowing the path of oil flow.
It would be desirable to be able to line a pipe with melt processible perfluoropolymer thereby creating an interior surface which has resistance to the corrosive effects of acids, and to which it is difficult for materials to adhere, which adheres well to the pipe, and which does not rely on the use of bonding agents or rotolining with fluoropolymer powders. Because of its resistance to cracking and failure, such a lining would be suitable for industries which require high purity. Further there is a desire that the lining be durable and adhere well to the pipe, and not likely to buckle, when used for many years for conveying corrosive materials.